1. Field of the Invention
The present invention relates to a technical field of a hydraulic pressure boosting apparatus of an open center type which uses the hydraulic pressure of hydraulic fluid to boost the leg power applied to a brake pedal of, for example, an automobile, so as to enlarge braking force.
The present application is based on Japanese Application Nos. Hei.9-332458 and 10-43513, which are incorporated herein by reference.
2. Description of the Related Art
A portion of vehicles, for example, automobiles, incorporates a hydraulic pressure boosting system for boosting operating force of an operating member by dint of the hydraulic pressure of hydraulic fluid so as to produce an output. The hydraulic pressure boosting system includes a hydraulic pressure boosting system arranged to obtain large braking force which cannot be obtained by only leg power applied to a brake pedal or to reduce the leg power which must be applied to the brake pedal. The hydraulic pressure boosting system incorporates a hydraulic pressure brake boosting apparatus which is operated by dint of the hydraulic pressure of hydraulic fluid so as to boost the leg power and operate a master cylinder.
As a hydraulic pressure boosting system of the foregoing type, a conventional hydraulic pressure boosting system has a structure as shown in FIG. 5. Referring to the drawing, reference numeral 1 represents a hydraulic pressure brake system, 2 represents a brake pedal, 3 represents an open center type hydraulic brake booster (hereinafter called a "brake booster" or simply called a "booster") which is operated by the brake pedal 2 and which boosts the leg power applied to the brake pedal 2 so as to produce an output, 4 represents a tandem type master cylinder which is operated by the output of the booster 3 and which generates braking hydraulic pressure, 5 represents a brake cylinder which is operated by dint of the braking hydraulic pressure applied from the master cylinder 4 so as to generate braking force which must be supplied to each wheel and 6 represents a pump which is operated by an engine 7 so as to supply hydraulic fluid to the booster 3. Reference numeral 8 represents a reservoir for accumulating the hydraulic fluid, and 9 represents an emergency accumulator for accumulating emergency hydraulic pressure for operating the booster 3 in a case where a breakdown or the like of the pump 6 results in inhibition of supply of the hydraulic fluid from the pump 6.
The open center type booster 3 incorporates a return valve, the gap of which is maximally opened when the brake is not operated so as to freely pass the hydraulic fluid. The gap of the return valve is throttled when the brake is operated so that the flow of the hydraulic fluid is limited and thus a hydraulic pressure is generated. The generated hydraulic pressure is used to boost the input so as to produce an output. A variety of structures have been known. For example, a known booster 3 is shown in FIG. 6.
As shown in FIG. 6, the booster 3 has a structure that a first annular groove 11, which is always communicated with the reservoir 8 through a circulating passage 10, and a second annular groove 13, which is always connected to the pump 6 through an inlet passage 12, constitute a return valve 14 which controls return of the hydraulic fluid discharged from the pump 6 to the reservoir 8. The second annular groove 13 and a third annular groove 20, which is always communicated with a power house 19, constitute a supply valve 21 for supplying the pressure generated in the second annular groove 13 and discharged from the pump 6 to the power house 19, the third annular groove 20 being communicated to the power house 19 through the second annular groove 13, a first radial-directional opening 15, an axial-directional opening 16, a second radial-directional opening 17 and a check valve 18. Moreover, the third annular groove 20 and a fourth annular groove 23 which is always communicated with the reservoir 8 through a discharge passage 22 constitute a discharge valve 24 for discharging, to the reservoir 8, the hydraulic fluid supplied to the power house 19.
In a state in which the brake is not operated as shown in FIG. 6, the return valve 14 is opened so that the gap (a gap between the first annular groove 11 and the second annular groove 13) of the return valve 14 is maximized. Moreover, the supply valve 21 is closed. In addition, the discharge valve 24 is opened so that a gap (a gap between the third annular groove 20 and the fourth annular groove 23) of the discharge valve 24 is maximized. Therefore, the hydraulic fluid discharged from the pump 6 passes through the inlet passage 12, the second annular groove 13, the return valve 14 and the circulating passage 10 of the open center type booster 3. Then, the hydraulic fluid is returned to the reservoir 8. Since the gap of the return valve 14 has been maximized in the above-mentioned case, substantially no hydraulic pressure is generated in the second annular groove 13.
When application of leg power to the brake pedal 2 causes the input shaft 25 to be moved forward in the above-mentioned state, the lever 26 is clockwise rotated around a fulcrum 27. Thus, the valve spool 28 is moved forward. Thus, the gap of the return valve 14 is throttled and the discharge valve 24 is closed. In addition, the supply valve 21 is opened. Since the gap of the return valve 14 is throttled (sometimes the return valve 14 is finally closed), hydraulic pressure is generated in the second annular groove 13. The generated hydraulic pressure is passed through the opened supply valve 21, the third annular groove 20, the first radial-directional opening 15, the axial-directional opening 16, the second radial-directional opening 17 and the check valve 18. Then, the hydraulic pressure is introduced into the power house 19. Since the hydraulic pressure introduced into the power house 19 acts on a power piston 29, the power piston 29 generates braking force obtained by boosting the leg power. An output of the braking force is produced from an output shaft 30 so that the master cylinder 4 is operated and thus the brake is operated.
The hydraulic pressure generated in the second annular groove 13 rightwards, in FIG. 6, moves a valve 33 of a charging valve 32 in the form of a check valve of an accumulator valve 31 so that the valve 33 is separated from a rubber seat 34 whereby opening charging valve 32. Thus, the hydraulic pressure passes through a gap between the valve 33 and the rubber seat 34, a portion around the valve 33 and an accumulator passage 35, and then introduced into the emergency accumulator 9 so as to be accumulated in the emergency accumulator 9.
When the brake pedal 2 has been released, the input shaft 25 is moved rearwards to a non-operating position shown in the drawing. Moreover, the lever 26 is counterclockwise rotated around the fulcrum 27 outer surface that the valve spool 28 is moved rearwards to a non-operating position shown in the drawing. As a result, the gap of the return valve 14 is opened maximally. Moreover, the supply valve 21 is closed and the discharge valve 24 is opened maximally. Therefore, the hydraulic fluid in the power house 19 is passed through the opening 17, 16 and 15, the third annular groove 20, the gap of the discharge valve 24, the fourth annular groove 23 and the discharge passage 22 so as to be discharged to the reservoir 8. As a result, the hydraulic pressure in the power house 19 is reduced. Thus, the power piston 29 is moved rearwards to a non-operating position shown in the drawing so that the braking force is vanished. Therefore, the master cylinder 4 is returned to the non-operating state so that the braking operation is suspended. Since the gap of the return valve 14 is maximized, the hydraulic pressure generated in the second annular groove 13 is vanished. Thus, the hydraulic fluid discharged from the pump 6 is passed through the return valve 14 and circulated to the reservoir 8 as described above.
FIG. 12 is a diagram showing an example of a conventional hydraulic booster. A power piston 108 is slidably engaged to a power cylinder 110 formed in a housing 106. A push rod 116 is connected to the front surface of the power piston 108 so that an output of the power piston 108 is transmitted to a master cylinder (not shown). An input cylinder 114 is formed on the same axis of the power cylinder 110. An input piston 112 is movably engaged to the inside portion of the input cylinder 114. An end 120b of the input rod 120 is secured to the front surface of the input piston 112. A leading end 120a of the input rod 120 is slidably received in a circular opening 108b formed on the rear surface of the power piston 108. A leading end ball portion of an operating rod which is movable when a brake pedal (not shown) is operated is connected to an end of the housing 106 of the input piston 112 facing outside. When the operating rod is moved, the input piston 112 and the input rod 120 are moved.
A spool valve 128 is provided for the inside portion of the housing 106, the spool valve 128 being arranged to introduce hydraulic fluid discharged from a pump 135 after a passage has been switched into a power pressure chamber 132 formed between the power piston 108 and the input piston 112. A spool 126 of the spool valve 128 is operably connected to the power piston 108 and the input rod 120 through a lever 146 so as to be moved when the input piston 112 and the input rod 120 are moved. Thus, the passage for the hydraulic fluid is switched.
An emergency accumulator 173 is connected to the hydraulic booster. When the pressure discharged from the pump 135 has been raised during the operation of the brake, a charge valve (not shown) is opened. Thus, the pressure is accumulated in the accumulator 173. If a boosting operation is not performed because of inhibition of rise in the pressure in the power pressure chamber 132 owing to a breakdown of the pump 135 or the like, the pressure in the accumulator 173 is discharged so that the boosting operation is performed. A dump valve 184 which is opened in an emergency or the like so as to introduce the pressure in the accumulator 173 into the power pressure chamber 132 is provided for the inside portion of the housing 106.
In general, the conventional dump valve 184 incorporates a seat member 186 including a passage, a ball valve 190 which can be placed on a valve seat of the seat member 186 and a pin 196 inserted into the passage in the seat member 186 and arranged to upwards push the ball valve 190 so as to separate the ball valve 190 from the valve seat in an emergency. The dump valve 184 is slidably engaged to an end of the spool 126 so as to be opened when a retainer 151 and a sleeve 152 urged toward the end by a spring 154 have been moved through the lever 146 to push the pin 196.
In the hydraulic booster having the above-mentioned structure, the operating rod is moved forwards (to the left in the drawing) when the brake pedal (not shown) has been pressed. Thus, the input piston 112 and the input rod 120 are moved forwards. When the input rod 120 has been moved forwards, the lever 146 is swung such that a connection pin adjacent to the spool 126 serves as a fulcrum to forwards move the power piston 108. The forward movement of the power piston 108 results in the push rod 116 to push the piston of the master cylinder so that pressure is generated in the master cylinder. When the pressure has been generated in the master cylinder, the forward movement of the power piston 108 is inhibited. Thus, the lever 146 starts swinging such that the connection pin connected to the power piston 108 serves as a fulcrum. Thus, the spool 126 is moved forwards. When the spool 126 has been moved forwards, the passage of the spool valve 128 is switched. As a result, the fluid discharged from the pump 135 is introduced into the power pressure chamber 132 so that the power piston 108 is operated and the boosting operation is performed.
In a case where the pump 135 has been broken, no pressure is supplied to the power pressure chamber 132 even if the input rod 120 is moved forwards to move the spool 126 to the left in FIG. 12 so as to switch the passage of the spool valve 128. When further forward movement of the spool 126 is inhibited after the full stroke of the spool 126, the rotation of the lever 146 causes the sleeve 152 and the retainer 151 to move forwards such that the spring 154 is compressed. Since the pin 196 of the dump valve 184 is pressed, the ball valve 190 is separated from the valve seat of the seat member 186 so that the dump valve 184 is opened. Thus, the pressure accumulated in the accumulator 173 is passed through the passage around the pin 196 so as to be supplied to the power pressure chamber 132. Therefore, the boosting operation is started.
In the booster 3 of the hydraulic pressure brake system 1, change in the velocity of the flow of the hydraulic fluid is enlarged when the valve spool 28 has been moved forwards and thus the gap of the return valve 14 has been throttled. Therefore, there is apprehension that fluid flow noise takes place.
Further, the conventional hydraulic booster has the structure that the dump valve is operated in an emergency to introduce the pressure accumulated in the emergency accumulator into the power pressure chamber. In this case, the hydraulic fluid in the accumulator is allowed to pass through a gap between the ball valve and the valve seat and the passage into which the pin has been inserted. Thus, the hydraulic fluid is introduced into the power pressure chamber. Therefore, there arises a problem in that noise of the flow of the fluid is caused because the velocity of the flow is raised since the hydraulic fluid is throttled by the gap and so forth.